To examine the relevance of angiotensin II type 1 receptor (AT1R) expression in the determination of myocardial function after ischemia/reperfusion, Sprague-Dawley rats were treated intravenously with antisense oligodeoxynucleotides (AS-ODNs) directed at AT1R mRNA (100 microg/rat, n=9) or scrambled antisense oligodeoxynucleotides (Scr-ODNs, 100 microg/rat, n=6). Both AS-ODNs and Scr-ODNs were given along with 300 microg/rat of liposome DOTAP/DOPE, a positive electron carrier (wt:wt= 1:1). The hearts from AS-ODN- or Scr-ODN-treated rats were excised 24 hours later, perfused in vitro, and subjected to 25 minutes of global ischemia followed by 30 minutes of reperfusion. Parallel groups of rats were given the specific AT1R antagonist losartan (10 mg/kg IV, n=6) or saline (n=7) 4 to 6 hours before excising the hearts. Ischemia/reperfusion resulted in a significant increase in myocardial AT1R expression (autoradiography and binding assay) and myocardial dysfunction, indicated by increases in coronary perfusion pressure and left ventricular end-diastolic pressure and a decrease in developed left ventricular pressure (all P<0.01 versus baseline) in the saline-treated group. AT1R protein and mRNA levels also increased in ischemic/ reperfused myocardial tissues. Administration of AS-ODNs or losartan, but not Scr-ODNs, preserved myocardial function and blocked the increased AT1R binding after ischemia/reperfusion (both P<0.01). Myocardial AT1R mRNA levels were not affected by either AS-ODNs or losartan, and the AT1R protein levels were significantly reduced by AS-ODN, but not losartan, treatment. Plasma angiotensin II levels increased after administration of losartan but not after administration of AS-ODNs. These observations imply a critical role of AT1R upregulation in determining myocardial function immediately after ischemia/reperfusion. AS-ODNs to AT1R mRNA may be more beneficial than losartan, because losartan does not affect the plasma angiotensin II level. The sustained increase in AT1R mRNA, but diminished protein expression, in rat hearts treated with AS-ODNs suggests that AS-ODNs block AT1R at the translational level.